The present invention relates to an image sensor array, such as found, for example, in digital scanners, copiers, and facsimile machines. More particularly, the present invention relates to a selectable amplifier which can be associated with at least one individual photosensor in such a sensor array.
Image sensor arrays typically comprise a linear array of photosensors which scan an image-bearing document and convert the microscopic image areas viewed by each photosensor to image signal charges. Following an integration time, the image signal charges are amplified and transferred to a common output line or bus through successively actuated multiplexing transistors.
In the scanning process, bias and reset charges are applied to each photosensor (such as a photodiode) in a predetermined time sequence during each scan cycle. In a particular embodiment of such an image sensor array, a two-stage transfer circuit is provided for transferring the image signal charges from the photodiodes. A bias charge is applied to each photodiode through a bias charge injection transistor coupled to a node between the photodiode and the input to the transfer circuit. From the transfer circuit, the image-based charges are caused to pass through an amplifier circuit, one amplifier circuit being typically provided for each photodiode, or at least to each RGB-color triplet of color-sensitive photodiodes.
During a readout of the image signals along an array of photodiodes, it is desirable that the individual amplifiers associated with each photodiode be activated only long enough to amplify the image signal being read out from a particular single photodiode; when the particular photodiode is not reading out its image based charge at the moment, it is desirable that the associated amplifier be powered down temporarily. This power-down is desirable from the perspective of lowering the total power requirements of a silicon chip which may have several hundred photodiodes and associated amplifiers thereon, as well as for other reasons.
The present invention is thus directed to a design of an individually-selectable amplifier which can be associated with a single photodiode or other photosensor in an image sensor array.
Bazes, xe2x80x9cTwo Novel Fully Complementary Self-Biased CMOS Differential Amplifiers,xe2x80x9d IEEE Journal of Solid-State Circuits, Vol. 26, No. 2, Feb. 1991, pp. 165-168, discloses designs of differential amplifiers having fully complementary configurations and which are self-biased through negative feedback. U.S. Pat. Nos. 4,857,476 and 4,958,133 by the same author show related amplifier designs.
U.S. Pat. No. 5,493,423, incorporated by reference above, discloses an amplifier circuit which can be associated with an individual photosensor in an image sensor array. With each cycle of passing an image signal through an amplifier, a low standby current is applied to certain transistors within the amplifier until the next signal is to be output. Critical nodes within the amplifier are caused to settle to known charge-values before each image signal is passed therethrough.
According to one aspect of the present invention, there is provided an amplifier suitable for processing image signals. An input stage includes a first differential pair including two p-devices and a second differential pair, complementary to the first differential pair, including two n-devices. A first load device supplies current to the first differential pair and a second load device supplies current to the second differential pair, the first load device and the second load device being biased at a common bias node. Self-biasing circuitry generates a bias at the bias node. Means are provided for deselecting the amplifier in response to an external signal, the deselecting means including means for grounding the bias node and means for eliminating current in the self-biasing circuitry in response to the external signal.
According to another aspect of the present invention, there is provided a photosensitive apparatus, comprising a set of photosensors, each photosensor outputting a voltage signal relating to an intensity of light thereon; a set of amplifiers, each amplifier in the set of amplifiers being associated with a photosensor in the set of photosensors, for amplifying a voltage signal from the photosensor; and means for sending a deselection signal to any amplifier. Each amplifier includes an input stage, the input stage having a first differential pair including two p-devices and a second differential pair, complementary to the first differential pair, including two n-devices. A first load device supplies current to the first differential pair and a second load device supplies current to the second differential pair, the first load device and the second load device being biased at a common bias node. Self-biasing circuitry generates a bias at the bias node. Means are provided for deselecting the amplifier in response to the deselection signal, the deselecting means including means for grounding the bias node and means for eliminating current in the self-biasing circuitry in response to the deselection signal.